MAINTAINERS: Switch maintenance for mrf24j40 driver over

Alan Ott has not been actively working on the driver or reviewing
patches for several years. I have been taking odd fixes in through the
wpan/ieee802154 tree. Update the MAINTAINERS file to reflect this
reality. I wanted to thank Alan for his work on the driver.

Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>
Link: https://lore.kernel.org/r/20230218211317.284889-3-stefan@datenfreihafen.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>